1.Field of the Invention
This invention relates to a developing roller for use in electrophotographic apparatus such as copying machines and printers and electrostatic recording apparatus for visualizing electrostatic latent images with a non-magnetic single-component developer as well as a developing apparatus using the same. More particularly, it relates to a developing roller which is least contaminative and ensures production of satisfactory images over a long period of use as well as a developing apparatus using the same.
2. Prior Art
In conjunction with electrophotographic apparatus such as copying machines and printers and electrostatic recording apparatus, one known developing process involving supplying a non-magnetic single-component developer to a photoreceptor having a latent image borne thereon to apply the developer to the latent image for visualizing the latent image is an impression developing process as disclosed in Schaffert, U.S. Pat. No. 3,152,012 and Bettiga et al., U.S. Pat. No. 3,731,146. Since this process eliminates a need for magnetic material, the apparatus can be readily simplified in structure, reduced in size, and modified to be compatible with color toners.
The impression developing process performs development by bringing a developing roller carrying a non-magnetic single-component developer or toner in contact with a latent image bearing member having a latent image borne thereon, typically a photoreceptor whereby the toner is transferred to the latent image on the latent image bearing member. Thus the developing roller must be made of a resilient material having conductivity.
Referring to FIG. 2, the impression developing process is described in detail. Between a toner applicator roller 4 for supplying a toner 6 and a photoreceptor 5 having a latent image borne thereon is disposed a developing roller 1 in close contact with the photoreceptor 5, but slightly spaced from the toner applicator roller 4. The developing roller 1, photoreceptor 5 and toner applicator roller 4 rotate in directions shown by arrows. The toner applicator roller 4 supplies the toner 6 to the surface of the developing roller 1 whereupon the toner 6 is leveled into a uniform layer by means of a distributing blade 7. As the developing roller 1 rotates in contact with the photoreceptor 5 in this state, the toner in a thin layer form is transferred from the developing roller 1 to the latent image on the photoreceptor 5 for visualizing the latent image. Also illustrated in FIG. 2 are a transfer section 8 for transferring the toner image from the member 5 to a recording medium such as a sheet of paper, and a cleaning section 9 including a cleaning blade 10 for removing the toner remaining on the surface of the photoreceptor 5 after transfer.
During rotation, the developing roller 1 must be kept in close contact with the photoreceptor 5. Then the developing roller 1 is of a structure shown in FIG. 1 as comprising a shaft 2 of a highly conductive material such as metal and a resilient sleeve 3 surrounding the shaft 2. The resilient sleeve 3 is made of a resilient material composed of an elastic rubber such as silicone rubber and NBR and a sponge material such as polyurethane foam and a conductive agent blended therein for imparting conductivity.
The prior art developing rollers have several drawbacks. The following two drawbacks are attributable to the resilient sleeve.
(1) Where the resilient sleeve is made of an elastic rubber such as silicone rubber and NBR, the rubber is reduced in hardness in order to ensure close contact, with the risk of contaminating the photoreceptor.
(2) Where the resilient sleeve is made of a sponge material such as polyurethane foam, the toner can penetrate into the resilient sleeve. Then over a long period of use, the developing roller becomes harder due to toner clogging and the toner experiences short charging, resulting in images of poor quality.
In general, the developing rollers having a single-layer resilient sleeve have (3) problems including an unsatisfactory rise of charging of the toner on the roller, fog in images due to toner short charging, poor cleaning due to selective development, a lowering of printing density, and a substantial lowering of printing quality after long-term continuous printing. These phenomena become more outstanding in a hot humid environment.
(4) If the resilient sleeve is less resistant against abrasion, the surface of the resilient sleeve is scraped and roughened by the distributing blade 7 (in FIG. 2). It is then difficult to provide a constant supply of toner onto the photoreceptor 5 (in FIG. 2) over a long term.
(5) The resilient sleeve can be deteriorated by ozone generating in copying machines and printers, giving rise to such inconvenience as cracks and increased compression set in the resilient sleeve. It is then difficult to provide a constant supply of toner onto the photoreceptor 5 (in FIG. 2) over a long term.
It was also proposed to form at least one conductive resin layer outside the resilient sleeve as disclosed in Japanese Patent Application Kokai (JP-A) No. 134468/1992. The developing roller having a conductive resin layer on the resilient sleeve suffers from the problems that the manufacturing process is complex because of a multi-layer structure, failing to provide a low cost developing roller and apparatus and that it is difficult to maintain the resistance of the conductive resin layer stable over a long period of time.